Hydraulic system with bi-rotational pump with filter title

ABSTRACT

An hydraulic system employing bi-rotational pumps for directing fluid under pressure to an actuator. In one embodiment the return fluid from one port of the actuator is filtered by means of a pilot operated check valve in a by-pass circuit leading to a filter and fluid reservoir, with an additional check valve being provided to preclude flow from the by-pass circuit to the associated port of the pump. In another embodiment return flow from the head end of the actuator is directed to a filter and reservoir by a two way control valve which also functions to direct flow from the pump to pressurize the head end of the actuator. A pilot operated check valve is provided to direct flow from the head end of the actuator directly to the reservoir even should the two way control valve be inoperable. In another embodiment duplex bi-rotational pump systems provide high volume flow rate at a relatively low pressure, or a low volume flow rate at a relatively high pressure. In one embodiment of the duplex bi-rotational pump system a circuit is provided to filter the fluid returning from the actuator to the reservoir by means of a two-way flow control valve.

United States Patent Gellatly et a].

Sept. 9, 1975 HYDRAULIC SYSTEM WITH BI-ROTATIONAL PUMP WITH FILTER TITLE[75] Inventors: Robert K. Gellatly, Los Gatos; Joe] B. Meredith, SanCarlos; Richard A. Green, Mountain View, all of Calif.

[73] Assignee: General Cable Corporation, San

Carlos, Calif.

[22] Filed: Dec. 5, 1974 [2]] Appl. No.: 529,601

Related US. Application Data [62] Division of Ser, No. 442,483, Feb. 14,1974, Pat No.

[52] U.S. CI. 60/453; 60/464; 60/468;

[51] Int. Cl. ..F1SB 21/04 [58] Field of Search 60/430, 453, 454, 46L

[56] References Cited UNITED STATES PATENTS 2,657,533 ll/l953 Schanzlinet al 60/468 X 3,303,647 2/1967 Futamata 60/468 X Primary Examiner-EdgarW. Geoghegan Attorney, Agent, or Firm-Flehr, Hohbach, Test, Albritton &Herbert 5 7 ABSTRACT An hydraulic system employing bi-rotational pumpsfor directing fluid under pressure to an actuator. In one embodiment thereturn fluid from one port of the actuator is filtered by means of apilot operated check valve in a by-pass circuit leading to a filter andfluid reservoir, with an additional check valve being provided topreclude flow from the by-pass circuit to the associated port of thepump. In another embodiment return flow from the head end of theactuator is directed to a filter and reservoir by a two way controlvalve which also functions to direct flow from the pump to pressurizethe head end of the actuator. A pilot operated check valve is providedto direct flow from the head end of the actuator directly to thereservoir even should the two way control valve be inoperable. Inanother embodiment duplex bi-rotational pump systems provide high volumeflow rate at a relatively low pressure, or a low volume flow rate at arelatively high pressure. In one embodiment of the duplex bi-rotationalpump system a circuit is provided to filter the fluid returning from theactuator to the reservoir by means of a two-way flow control valve.

I2 Claims, 6 Drawing Figures PATENTEI] SEP 91975 SHEET 1 BF 3 NE t Non3k aw PATENTEDSEP' ems SHEET 2 [IF 3 HYDRAULIC SYSTEM WITH BI-ROTATIONALPUMP WITH FILTER TITLE This is a division of application Ser. No.442,483, filed Feb. 14, 1974, now U.S. Pat. No. 3,864,911, granted Feb.I1, 1975.

BACKGROUND OF THE INVENTION This invention relates to hydraulic systemsfor delivering pressurized fluid to hydraulic actuators, and inparticular relates to systems of the type described which employbi-rotational pumps.

Various circuit designs have heretofore been developed for controllinghydraulic actuators, such as rotary or extensible hydraulic actuators orrams. Bi-rotational pumps have been developed for use in such hydrauliccircuits to reduce design complexity and cost. Conventionalbi-rotational pumps typically are of the intermeshing gear type whichare reversibly actuated with the ports of the pump alternating betweensuction and pressure functions so that an extensible actuator, forexample, can be either extended or retracted by driving the pump ineither direction.

Heretofore it has been difficult to adequately filter the hydraulicfluid in systems employing bi-rotational pumps because the return fluidfrom the actuator is returned to the suction face of the pump, with theresult that only the relatively small volume of fluid which is pumpedfrom the reservoir is filtered. In the case of a linear cylinder typeactuator, only the differential rod volume can be dumped to tank througha low micron pressure-line filter. Suction line filters must ofnecessity be limited to approximately 100 mesh screen to prevent pumpcavitation due to inadequate intake flow.

In many cases it has been found desirable to provide a dual stagepumping system in which one stage provides a high volume, low pressureflow to the actuator while another stage provides a low volume, highpressure flow. Hydraulic systems of the type used for elevating a boom,for example, advantageously employ such a dual stage system where in onestage the boom is lowered at a fast rate under a light load, and inanother stage the boom is raised under a heavy load. Conventional dualstage systems employ unloading valves to achieve this purpose, butheretofore a dual stage system of the type described has not beenproposed which makes it feasible to employ the advantages of abirotational pumping system.

OBJECTS AND SUMMARY OF THE INVENTION It is a general object of theinvention to provide a new and improved hydraulic system employingbi-rotational pumps for supplying pressurized fluid to operate hydraulicactuators.

Another object is to provide an hydraulic system employing bi-rotationalpumps in which there is provision for filtering of the flow of fluidreturning from one port of the actuator to the reservoir.

Another object is to provide a bi-rotational pump system of the typedescribed in which the return flow of fluid from one port of theactuator is blocked from the pump by a check valve and is directed to afilter through a valve in a by-pass circuit.

Another object is to provide a bi-rotational pump systern of the typedescribed which directs return flow from one port of the actuatorthrough a two way control valve to a filter and reservoir.

5 reservoir should the control valve become inoperable for any reason.

Another object is to provide a duplex bi-rotational pump system of thetype described which affords a first operating stage delivering lowvolume, high pressure 10 flow to the actuator, and another stagedelivering high volume, low pressure flow to the actuator.

Another object is to provide a duplex bi-rotational pump system of thetype described in which provision is made for the filtering of thereturn flow from one end 5 of the actuator through a two way flowcontrol valve.

Another object is to provide a duplex bi-rotational pump system of thetype described employing a two way flow control valve for return linefiltering in which provision is made to divert the return fluid directlyto the reservoir should the control valve become inoperable for anyreason.

The invention provides a hydraulic system employing bi-rotational pumpsfor operating hydraulic actuators. The ports of the pump are fluidlyconnected with the actuator by circuit means which includes a fluidreservoir, a suction circuit directing fluid from the reservoir to theports, a first supply circuit directing fluid from one of the pump portsto the actuator and a second sup ply circuit directing fluid from theother pump port to the actuator. In one embodiment return fluid from oneport of the actuator is blocked from the pump by a check valve and isdiverted to a filter and the fluid rescrvoir through a by-pass circuitunder control of a pilot operated valve operating responsive to thepressure in the first supply circuit. In another embodiment the re turnflow from one end of the actuator is diverted through the second circuitto the filter and reservoir by means of a two-way control valve. Shouldthe two-way control valve become inoperable for any reason the actuatorcan be operated in emergency situations through the provision of a pilotoperated valve which diverts the return flow in the second circuitdirectly to the reservoir. In another embodiment the hydraulic systememploys duplex, i.e. dual stage, bi-rotational pumps with means toreversibly co-actuate the pumps for directing pressurized fluid througheither of first and second supply circuits connected with separate portsof the actuator. For high volume, low pressure operation, flow from 50the two pumps is combined and directed to the actuator. For low volume,high pressure operation, an unloading valve diverts flow from a port ofthe first stage pump to the reservoir while a duplexing check valveprecludes flow from the second stage pump to the reservoir. During highvolume, low pressure operation means is provided to open the duplexingcheck valve responsive to pressure in the first supply circuit. Inanother embodiment of the duplex bi-rotational pump system return flowfrom one port of the actuator is blocked from the suction ports of thepumps and directed through a filter to the reservoir by means of atwo-way flow control valve. A pilot operated check valve is provided todivert return fluid directly to the reservoir to permit operation of theactuator should the flow control valve become inoperable for any reason.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of oneembodiment of the invention illustrating a bi-rotational pump systemhaving provision for return line filtration by means of a check valveand by-pass circuit;

FIG. 2 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a check valve and by-pass circuit;

FIG. 3 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a twoway control valve;

FIG. 4 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a twoway control valve;

FIG. 5 is a schematic diagram of another embodiment providing a duplexbi-rotational pump system; and

FIG. 6 is a schematic diagram of another embodiment providing a duplexbi-rotational pump system with provision for return line filtration bymeans of a two-way control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings FIG. 1illustrates generally at 10 an hydraulic system adapted for operating asuitable hydraulic actuator 11 such as a rotary actuator or extensiblecylinder or ram of the type used, for example, in the machine toolindustry.

Hydraulic system 10 includes a suitable bi-rotational pump 12,preferably of a design which incorporates intermeshing gears rotatablymounted within a pump housing. The pump is reversibly actuated by meansof a suitable motor 13, such as an electric motor, coupled with the pumpthrough a drive shaft 14. Rotation of drive shaft 14 by the motor in onerotational direction or first operation mode draws fluid under suctionfrom a first pump port 16 and directs the fluid under pressure throughthe other port 17. Rotation of the drive shaft in the oppositerotational sense or second mode reverses the fluid flow and draws fluidunder suction from the second port 17 and directs it under pressurethrough the first port 16.

Circuit means is provided for fluidly interconnecting the pump portswith the actuator. The circuit means includes a fluid reservoir 18connected with the pump ports through a bifurcated suction circuit 19.The fluid is drawn in this circuit through a suitable coarse filter 21,preferably of a size on the order of one hundred mesh, and is directedthrough a first branch 22 having a check valve 23 into the first port 16during the first mode of operation, and through a branch conduit 24through a check valve 25 into the second port 17 dur ing the second modeof operation. The respective check valves 23, 25 function to precludereturn flow into the reservoir through whichever branch is connectedwith the pressure face of the pump, depending upon the mode ofoperation.

The circuit means further includes a first supply circuit comprisingconduits 27, 28 which interconnect the first port of pump 12 with afirst port 29 of the actuator.

A second supply circuit is provided and includes conduits 31, 32, 33 forinterconnecting the second port 17 of the pump with the second port 34of the actuator. One-way valve means comprising a check valve 36 isprovided in conduit 31 of the second supply circuit to direct flowtherethrough only in a direction from the pump to the actuator.

A bifurcated return circuit is provided in the circuit means andcomprises first and second branch conduits 37, 38 connected withrespective first and second supply circuits and also connected with acommon conduit 39 leading to a fluid reservoir 41 through a suitablefine filter 42, preferably of a mesh size on the order of forty microns.While separate reservoirs 18, 41 are illustrated schematically, theinvention contemplates that the reservoirs could be interconnected, or asingle reservoir provided, for the suction and pressure relief circuits.Suitable pressure relief valves 45, 43 are provided in respective firstand second branch conduits of the return circuit. Preferably the reliefvalves comprise normally closed valves which sense pressure inrespective first and second supply circuits and open when the pressuresin the respective circuits exceed a predetermined value that is selectedto preclude overpressure damage to the system.

The circuit means further includes a by-pass circuit comprising aconduit 44 and pilot operated check valve 46 for directing fluid fromconduit 32 of the second supply circuit to the return circuit and filter42 during the second mode of pump operation in which the first supplycircuit is pressurized. One end of conduit 44 is connected in thecircuit on the outlet side of check valve 36, and the other end isconnected with the return circuit on the outlet side of pressure reliefvalve 43. A pilot conduit 47 connects pilot operated check valve 46 withconduit 28 of the first supply circuit for communicating the pressure ofthe first supply circuit to valve 46 for opening the same after suchpressure exceeds a predetermined value.

The use and operation of the hydraulic system of FIG. 1 will bedescribed assuming, for example, that actuator is an extensiblehydraulic ram having a rod end and a head end. The port 29 is at the rodend of the ram and the port 34 is at the head end. A valve having asetting to open at 800 psi is provided as the first pressure reliefvalve 45, and a valve having a setting to open at 1500 psi is providedas the second pressure relief valve 43. For extending the ram motor 13is energized to drive pump 12 in its first operational mode drawingfluid under suction through first port 16 and directing it underpressure through second port 17. Fluid is drawn into the first port fromreservoir 18 and coarse filter 2! through the branch of suction circuit22 and check valve 23. The pressurized fluid from second port 17 openscheck valve 36 and flows through conduits 32 and 33 of the second supplycircuit into the head end of ram. The check valves 25 and 46 andpressure relief valve 43 prevent pressurized fluid from returning toreservoir. Should an overload above l500 psi from the pressure face ofthe pump arise for any reason during this mode of operation relief valve43 will open to dump the fluid to reservoir 41.

For retracting the actuator motor 13 is actuated to reversely drive pump12 in its second mode in which fluid is drawn under suction throughsecond port 17 and directed under pressure through first port 16. Fluidis supplied to the second port from reservoir 18 and filter 21 throughbranch conduit 24 and check valve 25. Pressurized fluid from the firstport is directed through conduit 28 of the first supply circuit to therod end of the ram. Should an overload above 800 psi from the pressureface of the pump arise in this mode, relief valve 45 will open to dumpthe fluid to reservoir through conduit 37 of the return circuit. As theram retracts the pressure from the first supply circuit is communicatedthrough pilot conduit 47 to open pilot operated check valve 46. Thispermits return fluid from the head end of ram to flow through conduit 33of the second supply circuit into conduit 44 of the by-pass circuit andinto reservoir 41 through fine filter 42. The check valve 36 in thesecond supply circuit precludes this return flow from entering thesecond port of pump. 12.

FIG. 2 illustrates an embodiment similar to that of FIG. 1 providing anhydraulic system 50 employing a bi-rotational pump 51 driven by motor 52through drive shaft 53. The construction and operation of pump 51 andmotor 52 are similar to that described for the embodiment of FIG. 1.Circuit means is provided to fluidly interconnect respective first andsecond ports 54, 55 of the pump with the ports 57, 58 of an actuator 59,such as a rotary actuator or extensible hydraulic ram. The circuit meansinclude a bifurcated suction circuit having a conduit 61 drawing fluidfrom reservoir 62 through coarse filter 63 for supply through branchconduits 64, 65 and check valves 67, 68 to the pump ports.

A first supply circuit including conduits 70, 71 connects first port 54of the pump with port 57 of the actuator and a second supply circuitincluding conduits 72, 73 connects the second port of the pump with port58 of the actuator.

A return circuit is provided and includes a fine filter 74 leading toreservoir 76 from a first branch conduit 77 provided with a firstpressure relief valve 78 and a second branch conduit 79 provided with asecond pressure relief valve 81. Preferably the first and second reliefvalves comprise normally closed valves adapted to open responsive toupstream pressures in the respective supply circuits exceedingpredetermined values.

A by-pass circuit is provided to direct return fluid from port 58 of theactuator to fine filter 74 and reservoir 76 without entering the secondport 55 of the pump. The by-pass circuit includes a conduit 82 and pilotoperated check valve 83 normally precluding flow from the second supplycircuit to the return circuit. A pilot conduit 84 is connected tocommunicate pressure from the first supply circuit for opening checkvalve 83 when the first supply circuit pressure exceeds a predeterminedvalue. A one-way flow valve such as the check valve 86 is provided inconduit 73 to permit flow only in a direction from the pump through thesecond supply circuit. Check valve 86 is connnected in the circuit at apoint between the pump and the connection with the conduit leading tocheck valve 83 of the bypass circuit.

The operation of the hydraulic system of the embodiment of FIG. 2 issimilar to that described for the embodiment of FIG. 1. Assuming thatactuator 59 is an extensible hydraulic ram having its head end at port58 and its rod end at port 57, it is extended by actuating motor 52 toturn drive shaft 53 in a rotational sense for operating pump 51 in amode which draws fluid under suction through first port 54 anddischarges it under pressure through second port 55 and through thesecond supply circuit into port 58. Return flow from the rod end of theram is directed through conduit 71 where it combines with supply fluidfrom suction circuit conduit 64 for delivery into first port 54 of thepump. For retracting the ram motor 52 is actuated to reversely drive thepump to draw fluid from reservoir 62 and second port 55 and deliver itunder pressure into the first supply circuit and the rod end of the ram.Return fluid from the head end of the ram is directed through conduit 72where it is blocked from reaching the pump by check valve 86. Thepressure in the first supply circuit is simultaneously communicatedthrough pilot conduit 84 to open pilot operated check valve 83 whichdirects the return flow through return circuit 82 and to reservoir 76through fine filter 74. This permits the relatively large volume offluid from the head end of the ram to be filtered during its retractionstroke, and in addition the filtering action is positive in that thefluid is forced under pressure through the fine filter.

It will be realized that in the embodiment of FIG. 2 the second pressurerelief valve 81 functions to unload pump overpressure condition occuringin a portion of the second supply circuit between pump 51 and checkvalve 83. This is in comparison to the system in the embodiment of FIG.1 where the pressure relief valve 43 functions to relieve anoverpressure condition occurring in a portion of the second supplycircuit between the ram and check valve 36 under positive pump pres sureor back pressure from the ram.

FIG. 3 illustrates an embodiment providing an hydraulic system 90employing a bi-rotational pump 91 actuated through drive shaft 92 by asuitable reversible motor 93. Preferably the pump and motor are of thetype described above in connection with the embodiments of FIGS. 1 and2. That is, motor 93 is adapted to be actuated to turn the drive shaftin a rotational sense for operating pump 91 in a first mode which drawsfluid under suction through first port 94 and discharges it underpressure through second port 96, and in a reverse rotational sense tooperate the pump in a second mode drawing fluid under suction throughsecond port 96 and discharging it under pressure through first port 94.

Circuit means is provided for fluidly interconnecting the ports of thepump with a suitable actuator 97. The actuator is illustrated as anextensible hydraulic ram adapted to be mounted, as an example, forraising and lowering the boom of an aerial platform. The circuit meansincludes a fluid reservoir 98 and bifurcated suction circuit having acoarse filter 99 of a suitable size on the order of one hundred mesh, abranch conduit 101 having a check valve 102 permitting flow only in adirection toward the first port 94 of the pump, and a branch conduit 103having a check valve 104 directing flow only in a direction toward thesecond port 96.

A first supply circuit including conduits 105, 106 directs flow from thefirst port of the pump to port 107 at the rod end of the ram, and asecond supply circuit including conduits 108, 109 directs flow from thesecond port of the pump through a two-way control valve 111 to port 112at the head end of the ram.

A return circuit is provided and includes first and second branchconduits 113, 114 connected with respective first and second supplycircuits and also connected with a common conduit 116 leading to areservoir 1 17 through a fine filter 118 of a mesh size on the order offorty microns. Suitable pressure relief valves 119, 120 are provided inrespective first and second branch conduits and preferably comprisenormally closed valves. The valve 1 19 is adapted to open responsive tothe pressure in first supply circuits exceeding a predetermined value,and the valve 120 is adapted to open responsive to the pressure in thatportion of the second supply circuit between the pump and two-way valve111 exceeding a predetermined value.

Two-way control valve 111 controls the flow to and from the head end ofthe actuator. Preferably control valve 111 comprises a suitable spoolvalve normally urged by the force of pilot assisted, adjustable springservo unit 122 to the illustrated first position which interconnectsconduit 109 of the second supply circuit with a conduit 123 leading tothe conduit 114 in return circuit and reservoir 117 through filter 118.The spool of control valve 111 is urged to its second position by thedischarge pressure from pump port 96 communicated through pilot conduit124 into servo unit 126. In the second position of the spool controlvalve 111 establishes communication between conduit 108 and conduit 109in the second supply circuit to direct dis charge fluid from the pump toport 112 in the head end of the ram.

Pilot conduits 127, 128 communicate pressure from conduit 106 of thefirst supply circuit into servo unit 122 to act against the end of thespool and assist the spring force in urging the spool to its firstposition for directing return fluid through the filter 1 18 into thereservoir as the ram is retracting. The spring force urging the spool toits first position may be adjustably varied to selectively vary thepressure limit at which the spool is shifted to its second position forextending the ram.

A pilot operated check valve 129 is provided in a branch conduit 131connected between the suction circuit and conduit 108 of the secondsupply circuit. Check valve 129 normally precludes flow into the suctioncircuit through branch conduit 131, and is adapted to be openedresponsive to the pressure in the first supply circuit exceeding apredetermined value. The pressure in the first supply circuit iscommunicated to check valve 129 by pilot conduit 132 connected withpilot conduit 127. Check valve 129 serves to permit the return fluidfrom the head end of ram to flow directly to reservoir 98 should controlvalve 111 be inoperable and remain in its second position at the timepump 91 is actuated to pressurize the first supply circuit. This permitsthe boom to be lowered in an emergency situation where control valve 111fails to function properly.

The operation of the embodiment of FIG. 3 will be described assumingthat actuator 97 comprises an extensible ram mounted for elevating andlowering the boom of an aerial lift. First pressure relief valve 119 isselected to open responsive to a pressure in excess of 800 psi in thefirst supply circuit, and second relief valve 120 is selected to openresponsive to a pressure in excess of l500 psi in the second supplycircuit. For elevating the boom motor 93 is actuated to operate pump 91in its first mode drawing fluid under suction through the suctioncircuit and discharging it under pressure into conduit 108 of the secondsupply circuit. The pressure in conduit 108 communicates through pilotconduit 124 to shift the spool of control valve 1 1 1 to its secondposition to establish communication from the pump into conduit 109leading to the head end of ram. As the ram extends return fluid from itsrod end is directed through the first supply circuit and into conduitwhere it combines with flow from the suction circuit to supply fluidinto first port 94 of the pump.

For retracting the actuator to lower the boom the motor is reverselyactuated to operate pump 91 in its second mode drawing fluid underpressure through second port 96 and discharging it under pressurethrough first port 94 into the first supply circuit and the rod end ofthe ram. The pressure in the first supply circuit is communicatedthrough pilot conduits 127 and 128 to assist the spring in servo 122 toshift the spool of control valve 111 to its illustrated first position.In this position the return flow from the head end of the ram isdirected into conduit 123 of the return circuit and through fine filter118 into reservoir 117. It will be realized that a relatively largevolume of fluid is filtered in this manner on the retraction stroke ofthe ram, and in addition the fluid is forced through the filter underpressure from the ram.

FIG. 4 illustrates another embodiment providing an hydraulic system 136similar to the embodiment of FIG. 3. This system incorporates abi-rotational pump 137 operated through drive shaft 138 by reversiblemotor 139. The pump and motor are similar to that described inconnection with the foregoing embodiments.

Circuit means is provided for fluidly interconnecting the first andsecond ports 141, 142 of pump 137 with a suitable actuator 143, whichpreferably comprises an extensible ram. The circuit means includes asuction circuit having a reservoir 144 for supplying fluid to the pumpports through a coarse filter 146. A check valve 147 in one branch 148of the suction circuit permits flow only in a direction toward the firstport of the pump, and a check valve 149 in the other branch 151 permitsflow only in a direction toward the second port of the pump. A firstsupply circuit having conduits 152, 153 directs flow from the first portof the pump to the rod end of the ram, and a second supply circuithaving a conduit 154 directs flow from the second port through a two-waycontrol valve 156 and conduit 157 to the head end of the ram.

A return circuit is provided to direct return fluid through a conduit158, a fine filter 159 and into a reservoir 160. A first branch 161 ofthe return circuit is provided with a first pressure relief valve 162adapted to open responsive to pressure in the first supply circuitexceeding a predetermined value, and a second branch 163 is providedwith a second pressure relief valve 164 adapted to open responsive tothe pressure in the second supply circuit exceeding anotherpredetermined value.

Two-way control valve 156 preferably comprises a spool valve normallyurged to its first position, as illustrated in FIG. 4, by the force ofpilot assisted adjustable spring servo unit 166. The pressure of secondsupply conduit 154 is communicated to servo 166 through a pilot conduit167. In its first position the spool directs discharge fluid from thepump into conduit 157 leading to the head end of ram. The spool isactuated to its second position by the pressure of first supply conduit153 communicated through pilot conduits 168, 169 into servo unit 171. Inits second position the spool directs return fluid from the head end ofram into return conduit 158 and through the fine filter 159 to reservoir160.

A branch conduit 172 and pilot operated check valve 173 are providedbetween the suction circuit and conduit 154 in the second supplyconduit. Check valve 173 normally precludes flow from the second supplyconduit into the suction circuit and is opened responsive to apredetermined pressure in the first supply conduit communicated throughpilot conduits 168 and 174. With check valve 173 open in this mannerreturn fluid from the head end of the ram is diverted to reservoir 144should control valve 156 be inoperable with its spool valve lodged inthe first position as the ram is retracted.

In operation of hydraulic system 136 it will be assumed that actuator143 is an hydraulic ram mounted to raise and lower the boom of an aeriallift. First relief valve 162 is set to open responsive to the pressurein the first supply circuit exceeding 800 psi, and second relief valve164 is set to open responsive to the pressure in the second supplycircuit communicating with the pump discharge exceeding 1500 psi. Forextending the actuator to raise the boom motor 139 is actuated tooperate pump 137 in its first mode drawing fluid under suction throughfirst port 141 and discharging it under pressure into the second supplycircuit. With the spool of the control valve 156 urged to its firstposition by the spring force of servo 166 assisted by the pressure inpilot conduit 167, pressurized fluid from the pump is directed intoconduit 157 and the head end of the ram. Return fluid from the rod endis directed through the first supply circuit into conduit 152 where itcombines with flow through branch conduit 148 of the suction circuit tosupply the suction side of the pump.

For retracting the ram to lower the boom, motor 139 is reverselyactuated to operate pump 137 in its second mode drawing fluid from thesuction circuit and discharging it under pressure into conduit 153 ofthe first supply circuit and the rod end of the ram. The pressure inconduit 153 is communicated through pilot conduits 168 and 169 to shiftthe spool of valve 156 to its second position. Return fluid from thehead end of the ram then flows through conduit 157 and valve 156 into return conduit 158 and through fine filter 159 into the reservoir. Thus, arelatively large volume of fluid is filtered on the retraction stroke ofthe ram, and moreover the fluid is forced under pressure of the ramthrough the fine filter.

FIG. illustrates another embodiment providing an hydraulic system 176incorporating a pair of birotational first and second stage pumps 177,178 connected to provide duplex, i.e. two-stage, operation. In one stageflow from the two pumps is combined for high volume, low pressureoperation, and in another stage one pump is unloaded for low volume,high pressure operation. The pumps preferably are of the intermeshinggear pump type as described in the foregoing embodiments, and aremounted for reversible coactuation on a drive shaft 179 powered byreversible motor 181, such as an electric motor. Motor 181 is adapted toconjointly actuate the two pumps in a first mode for drawing fluid undersuction through their respective first ports 182, 183 and discharging itunder pressure through their respective second ports 184, I85, and in asecond mode drawing fluid under suction through their second ports anddischarging it under pressure through their first ports.

Circuit means is provided for fluidly interconnecting the ports of thepumps with a suitable actuator 187, illustrated as an extensiblehydraulic ram. The circuit means include a suction circuit having aconduit 188 drawing fluid from a reservoir 189 through a coarse filter191 of a size on the order of one hundred mesh. A branch 192 of thesuction circuit directs flow through a check valve 193 and conduits 194,195 and 196 leading to the first ports of the pumps, and another branchdirects flow through a check valve 198 and conduits 199, 200 and 401leading to the second ports of the pumps. Conduit 403 provides a firstsupply circuit directing flow to the rod end of the actuator, andconduit 204 provides a second supply circuit directing flow to the headend of the actuator. A bifurcated return circuit including conduit 205is provided to direct return flow to reservoir 189 through a fine filter201 of a mesh size on the order of forty microns. A first branch 202 ofthe return circuit inncludes a pressure relief valve 203 and isconnected with the first supply circuit. A second branch 204 of thereturn circuit is provided with a second pressure relief valve 206 andis connected through a conduit 207 with conduit 204 of the second supplycircuit. Preferably the pressure relief valves are normally closedvalves adapted to open responsive to the upstream pressures in thecircuits exceeding predetermined values.

Valve means for unloading the first stage pump 177 when the pressure inthe second supply circuit exceeds a predetermined value it is providedand comprises an unloading valve 208 in a branch conduit 209 connectedwith the conduit 401 leading from the second port of the first stagepump, with the outlet of the unloading valve dumping fluid intoreservoir 189. Preferably unloading valve 208 comprises a normallyclosed valve sensing pressure in the second supply circuit throughconduit 207 and a conduit 211, with the valve being set to open whensuch pressure exceeds a predetermined value.

Duplexing valve means is provided to preclude flow from the second stagepump to the reservoir through the open unloading valve during the lowvolume, high pressure stage, and to permit suction flow to the firststage pump during the high volume, low pressure stage. The duplexingvalve means comprises a pilot operated check valve 212 in a conduit 213interconnecting the conduits leading to the second ports of the twopumps. The check valve normally permits flow from the first stage pumpto combine with that from the second stage pump for high volume, lowpressure supplied to the head end of the actuator. A higher pressure inconduit 213 of the second supply circuit when unloading valve 208 isopen functions to close the duplexing check valve so that a low volume,high pressure from second stage pump 178 is delivered to the head end ofthe actuator. During the low pressure stage duplexing check valve 212opens responsive to a given pressure in the first supply circuit sensedthrough a pilot conduit 214 to permit suction fluid to be supplied topump 177 from check valve 198, conduits 197, 188 and reservoir 189.

A difi'erential volume valve comprising a pilot operated check valve 216is provided in a conduit 217 connected between conduit 204 of the secondsupply circuit and conduit 197 of the suction circuit. The valve 216functions to divert a portion of the greater volume of flow from thehead end of the actuator during retraction into the suction circuit.This valve is opened responsive to the pressure in first supply circuitas sensed through a pilot conduit 218, exceeding a predetermined value.

ln operation, it will be assumed that actuator 187 comprises anextensible ram mounted to raise and lower the boom of an aerial lift.The bi-rotational pumps 177, 178 are each sized to produce a l.5 gpmflow rate with motor 181 operating at 3450 rpm, thereby providing at 2:lflow ratio between low and high pressure modes. As desired, the pumpscould be sized to provide different flow ratios, for example a 3:1 ratiobetween the low and high pressure modes. First pressure relief valve 203is set to open responsive to pressure in the first supply circuitexceeding 850 psi and second pressure relief valve 206 is set to openresponsive to pressure in the second supply circuit exceeding I500 psi.The unloading valve 208 is set to open responsive to pressure in thesecond supply circuit exceeding 1000 psi.

For extending the ram to raise the boom, motor 181 is energized toconjointly drive the first and second stage pumps in a mode drawingfluid under suction through their respective first ports 182, 183 anddischarging it under pressure through second ports 184, I85. Fluid fromfirst stage pump 177 is diucted through duplexing check valve 212 andthe second supply circuit where it combines with the discharge flow fromsecond stage pump 178 for delivery to the ram through conduit 204. Asthe ram extends and the load on the boom increases, the dual stage pumpdelivery mode is maintained until a pressure of lOOO psi is exceeded inthe second supply circuit. This pressure is communicated throughconduits 207 and 211 to open unloading valve 208. The discharge fromfirst stage pump 177 then dumps through the unloading valve to thereservoir. This releases the upstream pressure on duplexing check valve212 so that the pressure from second stage pump 178 closes the duplexingvalve preventing second stage flow from discharging to reservoir. Thesystem thus is capable of operating the ram under high pressure, lowflow conditions utilizing relatively lower horsepower requirements ascompared to that which would be required by utilizing the flow from twopumps. As the ram extends return flow from its rod end is directedthrough conduit 403 of the first supply circuit where it combines withthe intake flow from check valve 193 of the suction circuit for deliveryto the first and second stage pumps.

For retracting the actuator to lower the boom, motor 181 is energized toreversely and conjointly drive the two pumps in a low pressure, highvolume mode drawing fluid under suction through their second ports 184,I85 and discharging it under pressure into conduit 203 of the firstsupply circuit and the rod end of the actuator. Pressure in the firstsupply circuit is communicated through pilot conduit 214 to openduplexing check valve 212. This permits intake fluid from check valve198 of the suction circuit to flow through the duplexing check valve andsupply the second port 184 of the first stage pump. Fluid supply to bothpumps in this mode comes from the head end of actuator 187 throughconduits 204, 199, 213, 200 and 401 to pump intake ports 185 and 184.

During the low pressure, high volume mode for retracting the actuator,pressure from the first supply circuit is communicated through pilotconduit 218 to open pilot operated check valve 216. This permits thedifferential volume between the head and rod ends of actuator 187 toflow through this valve and discharge to reservoir 189 through conduits217, 197 and 188 and filter 191.

FIG. 6 illustrates an embodiment of the invention providing an hydraulicsystem 220 incorporating birotational pumps 22] and 222 arranged toprovide duplex, i.e. two-stage, flow to an actuator 223 with provisionfor filtration of return fluid from one end of the actuator. The firststage pump 221 and second stage pump 222 are mounted for conjointreversible operation through a drive shaft 224 from a suitable motor226, such as an electric no tor. The two pumps preferably are of thepreviously described gear pump type and are adapted for operationbetween one mode drawing fluid under suction through their respectivefirst ports 227, 228 and discharging it under pressure through theirrespective second ports 229, 230 and a second mode drawing fluid undersuction through their second ports and discharging it under pressurethrough their first ports.

Circuit means is provided for fluidly interconnecting the ports of thepump with actuator 223, shown as an extensible ram. A suction circuitdirects fluid from reservoir 232 through a coarse filter 233, of a sizeon the order of one hundred mesh, through a first branch conduit 234with check valve 236 and a second branch conduit 237 with check valve238. Fluid from the first branch conduit is supplied through conduit 239to the first port of second stage pump 222 and through conduits 241, 242to the first port of first stage pump 221. Fluid from the second branchconduit 237 is supplied to the second port of the second stage pumpthrough conduit 243, and through duplexing check valve 244 into conduit246 to the second port of the first stage pump. A first supply circuitincluding a conduit 247 is provided to direct the flow from the firstports of the pumps to the rod end of the actuator, and a second supplycircuit including a conduit 248 is provided to direct flow from thesecond ports of the pumps through a twoway control valve 249 to the headend of the actuator.

A return circuit including conduits 251 and 252 is provided to directreturn fluid to the reservoir through a fine filter 253 of a mesh sizeon the order of forty microns into reservoir 254. A first branch conduit256 having a pressure relief valve 257 and a second branch conduit 258having a second pressure relief valve 259 are provided to relieveoverpressure from the supply circuits. Preferably the pressure reliefvalves comprise nonnally closed valves adapted to open responsive to theupstream pressure in respective supply circuits exceeding apredetermined value.

Unloading valve means is provided and comprises a normally closed pilotoperated valve 261 in a conduit 262 connected between the first stagepump outlet conduit 246 and the return circuit. A pilot conduit 263 isprovided to communicate second stage pump discharge pressure fromconduits 243 and 264 for opening unloading valve 261 responsive to suchpressure exceeding a predetermined value.

The duplexing check valve 244 comprises a pilot operated check valvewhich normally precludes flow from the second stage pump to reservoir254 when unloading valve 261 is open during the high pressure operatingmode. Pilot conduits 266 and 267 communicate pressure from the firstsupply circuit to open the duplexing check valve permitting fluid to besupplied to second port 229 of the first stage pump when the pumps areoperating in their low pressure mode for retracting the actuator.

Two-position flow control valve 249 preferably comprises a spool valvenonnally urged by pilot pressure assisted spring servo unit 301 to itsillustrated first position for establishing communication betweenconduit 248 connected with the head end of the actuator and returncircuit conduit 25], filter 253, and reservoir 254. A pilot conduit 302communicates pressure from the first supply circuit to servo 301 toassist in shifting the spool to its first position when the rod end ofthe actuator is pressurized. The spool is shifted to its second positionby servo unit 303 responsive to the pressure communicated through pilotcondut 304 of discharge fluid from the second port of pump 222. In thesecond position of the spool the pump discharge is directed to the headend of the actuator.

A pilot operated check valve 306 is provided in a conduit 307 connectedbetween conduit 243 and the suction circuit. This check valve normallyprecludes flow from the discharge of the second stage pump to thesuction circuit, and is opened responsive to a predetermined pressurecommunicated from the first supply circuit through pilot conduits 266and 308. The function of check valve 306 is to insure that the actuatorcan be retracted should control valve 249 become inoperable with itsspool lodged in its second position. If the control valve does fail tomove from its second position when a retraction stroke is desired, checkvalve 306 will open as the pumps pressurize the first supply circuit,and return fluid from the head end of the actua tor is then directedthrough the control valve and diverted through check valve 306 to thesuction circuit.

In operation it will be assumed that actuator 223 comprises an hydraulicram mounted to raise and lower the boom of an aerial lift. Thebi-rotational pumps 221, 222 are each sized to deliver 1.5 gpm flow whendriven at 3450 rpm by motor 226. First pressure relief valve 257 is setto open responsive to first supply circuit pressure exceeding 850 psi,and second relief valve 259 is set to open responsive to second supplycircuit pressure exceeding 2000 psi. Unloading valve 261 is set to openresponsive to the second stage pump discharge pressure from conduit 243exceeding 1200 psi.

For extending the actuator to elevate the boom, motor 226 is energizedto conjointly operate the first and second stage pumps in a mode drawingfluid under suction through the first ports and discharging it underpressure through their second ports. Fluid from first stage pump 221flows through duplexing check valve 244 and combines with flow from thesecond stage pump to communicate pressure through pilot conduit 304 toshift the spool of control valve 249 to its second position, therebydirecting combined flow from the two pumps through conduit 248 to thehead end of the actuator. Suction fluid is supplied to the two pumpsfrom reservoir 232, through filter 233, check valve 236 and conduits239, 241, 242. Return fluid from the rod end of the actuator is directedthrough conduit 247 to combine with the flow into the suction ports ofthe two pumps. This operational mode is maintained until the pressure inthe second supply circuit exceeds 1200 psi, at which point unloadingvalve 261 is opened due to the pressure communicated through pilotconduit 263. The discharge from first stage pump 221 then flows throughthe unloading valve and filter 253 to reservoir 254. The unloading ofthe discharge from the first stage pump reduces pressure in the freedirection on check valve 244 causing it to close, thereby preventingdischarge from the second stage pump from reaching the reservoir. Thesecond stage pump then continues to operate to supply low volume, highpressure flow to the head end of the actuator.

For retracting the actuator to lower the boom, motor 226 is reversed todrive the two pumps in a mode discharging fluid under pressure throughtheir first ports through conduit 247 to the rod end of the actuator.Pressure in the first supply circuit is communicated through pilotconduit 302 to servo unit 301 to assist the spring force in shifting thespool valve to its first posi tion, thereby directing return flow fromthe head end of the actuator to return conduit 251, filter 253 andreservoir 254. Pressure in the first supply circuit is also communicatedthrough pilot conduits 266 and 267 to open duplexing check valve 244.This permits suction fluid to flow from conduit 264 into second port 229of the first stage pump. First circuit pressure also opens check valve306 through pilot conduit 266, thereby permitting the differentialvolume fluid which is not supplied to the suction ports of the pumps tobe discharged through conduit 307 to reservoir 232 through filter 233 inthe event that valve 249 sticks in its second position.

While the foregoing embodiments are at present considered to bepreferred it will be understood that variations and modifications may bemade therein by those skilled in the art and it is intended to cover allsuch variations and modifications as fall within the true spirit andscope of the invention as expressed in the appended claims.

We claim:

l. in a system for operating an hydraulic actuator, the combination of areversible pump having first and second ports each of which is capableof assuming pressure and suction functions depending upon the directionof operation of the pump, fluid circuit means for directing fluidbetween the ports of the pump and said actuator, said circuit meansincluding fluid reservoir means, a suction circuit to direct fluid fromthe reservoir means to said ports, a first supply circuit 6 direct fluidunder pressure from the first port to said actuator, a second supplycircuit to direct fluid under pressure from the second port to saidactuator, one-way valve means in said second supply circuit to permitfluid flow therethrough only in a direction from said pump, a returncircuit to direct return fluid from the actuator to the reservoir means,filter means for filtering fluid in the return circuit, and by-passcircuit means for directing fluid from the portion of said second supplycircuit in the check flow direction of said one-way valve means throughthe return circuit and through the filter means responsive to pressurein said first supply circuit exceeding a given predetermined value.

2. A system as in claim 1 in which said by-pass circuit means includespilot operated valve means for normally closing said by-pass circuit andfor opening the same responsive to pressure in said first supply circuitexceeding said given predetermined value.

3. A system as in claim 1 in which said by-pass circuit means includespilot operated check valve means normally precluding flow therethroughin a direction toward said return circuit together with a pilot conduitconnected with said first supply circuit to communicate pressure thereinto said pilot operated check valve for opening the same responsive tosaid first supply circuit pressure exceeding said given predeterminedvalue.

4. A system as in claim 1 in which said return circuit includes reliefvalve means connected with said second supply circuit downstream of saidone-way valve means in its free flow directions for directing fluid fromthe second supply circuit to the return circuit and filter meansresponsive to the pressure in the second supply circuit exceeding asecond predetermined value.

5. A system as in claim 1 in which said return circuit includes reliefvalve means connected with said second supply circuit upstream of saidone-way valve means in its free flow direction for directing fluid fromthe second supply circuit to the return circuit and filter meansresponsive to the pressure in said second supply circuit exceeding saidsecond predetermined value.

6. A system as in claim 1 which includes means for actuating said pumpin a selected direction of operation for pumping fluid under pressureinto a selected first or second supply circuit to the actuator.

7. In a system for operating an extensible hydraulic actuator having ahead end and a rod end, the combination of a reversible pump havingfirst and second ports each of which is capable of assuming pressure andsuction functions depending upon the direction of operation of the pump,fluid circuit means for directing fluid between the ports of the pumpand said actuator, said circuit means including fluid reservoir means, asuction circuit to direct fluid from the reservoir means to said ports,a first supply circuit to direct fluid under pressure from the firstport to the rod end of said actuator, a second supply circuit to directfluid under pressure from the second port to the head end of saidactuator, a return circuit to direct fluid from the actuator to thereservoir means, filter means for filtering fluid in the return circuit,and two-way control valve means in the second supply circuit operablebetween a first condition directing return fluid under pressure from thehead end of the actuator through the return circuit and through thefilter means and a second condition directing fluid from the second portto the head end of the actuator.

8. A system as in claim 7 which includes pilot operated valve means todirect fluid from the portion of said second supply circuit between thepump and control valve means to said reservoir means responsive to thepressure in the first supply circuit exceeding an establishedpredetermined value.

9. A system as in claim 8 in which said return circuit includes firstrelief valve means to direct fluid from the first supply circuit to thereservoir means responsive to the pressure in the first supply circuitexceeding a first predetermined value and second relief valve means todirect fluid from the portion of the second supply circuit between thepump and control valve means to the reservoir means responsive topressure in said portion exceeding a second predetermined value greaterthan said first value.

10. A system as in claim 7 in which said control valve means is normallyoperated to its first condition and is operated to its second conditionresponsive to pressure of the fluid discharging from the second portexceeding a given predetermined value.

11. A system as in claim 7 in which said control valve means is normallyoperated to its second condition and is operated to its first conditionresponsive to pressure of the fluid in said first supply circuitexceeding a given predetermined value.

12. A system as in claim 7 which includes means for actuating said pumpin a selected direction of operation for pumping fluid under pressureinto a selected first or second supply circuit to the actuator.

1. In a system for operating an hydraulic actuator, the combination of areversible pump having first and second ports each of which is capableof assuming pressure and suction functions depending upon the directionof operation of the pump, fluid circuit means for directing fluidbetween the ports of the pump and said actuator, said circuit meansincluding fluid reservoir means, a suction circuit to direct fluid fromthe reservoir means to said ports, a first supply circuit to directfluid under pressure from the first port to said actuator, a secondsupply circuit to direct fluid under pressure from the second port tosaid actuator, one-way valve means in said second supply circuit topermit fluid flow therethrough only in a direction from said pump, areturn circuit to direct return fluid from the actuator to the reservoirmeans, filter means for filtering fluid in the return circuit, andby-pass circuit means for directing fluid from the portion of saidsecond supply circuit in the check flow direction of said one-way valvemeans through the return circuit and through the filter means responsiveto pressure in said first supply circuit exceeding a given predeterminedvalue.
 2. A system as in claim 1 in which said by-pass circuit meansincludes pilot operated valve means for normally closing said by-passcircuit and for opening the same responsive to pressure in said firstsupply circuit exceeding said given predetermined value.
 3. A system asin claim 1 in which said by-pass circuit means includes pilot operatedcheck valve means normally precluding flow therethrough in a directiontoward said return circuit together with a pilot conduit connected withsaid first supply circuit to communicate pressure therein to said pilotoperated check valve for opening the same responsive to said firstsupply circuit pressure exceeding said given predetermined value.
 4. Asystem as in claim 1 in which said return circuit includes relief valvemeans connected with said second supply circuit downstream of saidone-way valve means in its free flow directions for directing fluid fromthe second supply circuit to the return circuit and filter meansresponsive to the pressure in the second supply circuit exceeding asecond predetermined value.
 5. A system as in claim 1 in which saidreturn circuit includes relief valve means connected with said secondsupply circuit upstream of said one-way valve means in its free flowdirection for directing fluid from the second supply circuit to thereturn circuit and filter means responsive to the pressure in saidsecond supply circuit exceeding said second predetermined value.
 6. Asystem as in claim 1 which includes means for actuating said pump in aselected direction of operation for pumping fluid under pressure into aselected first or second supply circuit to the actuator.
 7. In a systemfor operating an extensible hydraulic actuator having a head end and arod end, the combination of a reversible pump having first and secondports each of which is capable of assuming pressure and suctionfunctions depending upon the direction of operation of the pump, fluidcircuit means for directing fluid between the ports of the pump and saidactuator, said circuit means including fluid reservoir means, a suctioncircuit to direct fluid from the reservoir means to said ports, a firstsupply circuit to direct fluid under pressure from the first port to therod end of said actuator, a second supply circuit to direct fluid underpressure from the second port to the head end of said actuator, a returncircuit to direct fluid from the actuator to the reservoir means, filtermeans for filtering fluid in the return circuit, and two-way controlvalve means in the second supply circuit operable between a firstcondition directing return fluid under pressure from the head end of theactuator through the return circuit and through the filter means and asecond condition directing fluid from the second port to the head end ofthe actuator.
 8. A system as in claim 7 which includes pilot operatedvalve means to direct fluid from the portion of said second supplycircuit between the pump and control valve means to said reservoir meansresponsive to the pressure in the first supply circuit exceeding anestablished predetermined value.
 9. A system as in claim 8 in which saidreturn circuit includes first relief valve means to direct fluid fromthe first supply circuit to the reservoir means responsive to thepressure in the first supply circuit exceeding a first predeterminedvalue and second relief valve means to direct fluid from the portion ofthe second supply circuit between the pump and control valve means tothe reservoir means responsive to pressure in said portion exceeding asecond predetermined value greater than said first value.
 10. A systemas in claim 7 in which said control valve means is normally operated toits first condition and is operated to its second condition responsiveto pressure of the fluid discharging from the second port exceeding agiven predetermined value.
 11. A system as in claim 7 in which saidcontrol valve means is normally operated to its second condition and isoperated to its first condition responsive to pressure of the fluid insaid first supply circuit exceeding a given predetermined value.
 12. Asystem as in claim 7 which includes means for actuating said pump in aselected direction of operation for pumping fluid under pressure into aselected first or second supply circuit to the actuator.